This add-on is not compatible with your version of Firefox because of the following:

W3V8, based on the IBM Intranet style (W3 v8.0) and Nautipolis (with icons from art.gnome.org).Check out the Tools/Options dialog!Includes support for many extensions, icluding downloadstatusbar, tabsidebar, all-in-sidebar, stumble-upon, downloadmg...